Field of the Invention
This invention relates to a separator serving as a constituent member of a fuel cell as well as a cell stack obtained by combining constituent members that include these separators.
Description of the Related Art
FIG. 35 illustrates the basic structure of single cell constituting a conventional polymer electrolyte fuel cell (PEFC).
A single cell 90 is constructed by bringing ribbed separators (RS) 98 into pressing contact with respective ones of both sides of a membrane electrode assembly (MEA) 91. The membrane electrode assembly (MEA) 91 is arranged and integrated by pressure-bonding or hot-pressing an anode 93 and a cathode 94 onto respective ones of both sides of a polymer electrolyte membrane 92 (PEM). The anode 93 and cathode 94 each comprise a catalyst layer (CL) 95, a porous (microporous) layer (MPL) 96 and a gas diffusion layer (GDL) 97. Since the output voltage of the single cell theoretically is a maximum of 1.2V, a high output voltage is obtained by stacking such single cells.
The separators 98 function to achieve electrical connection to the + electrode (cathode) and − electrode (anode) of adjoining single cells and to supply a cathode gas (air, oxygen) and an anode gas (fuel, hydrogen) to respective ones of both electrodes from gas flow paths provided in the separator surfaces.
Typically the gas diffusion layer (GDL) 97 is composed of carbon paper or woven or non-woven cloth of carbon fibers that has been rendered partially water-repellant using fluorocarbon resin (PTFE) or the like. Typically the porous layer (MPL) 96 is a porous layer comprising fine particles of carbon rendered suitably water-repellant (or hydrophilic) and controlled in terms of pore diameter and functions to form a catalyst layer of uniform thickness, to supply reactant gas to the catalyst reaction layer or to perform smooth mass transfer of catalyst reaction product (water that has been produced). Further, the catalyst layer (CL) 95 is obtained by coating the surface of the electrolyte (PEM) or the surface of the porous membrane (MPL) with a catalyst (Pt/CB) having nanometer-sized particles of platinum supported in highly dispersed fashion on a carrier of fine carbon particles, the coating being achieved using an electrically conductive ionomer (such as Nafion) as a binder. The reactivity of the catalyst per se, in particular the oxygen-reduction reactivity (ORR), and the rate at which oxygen and protons are supplied to the catalyst layer are important factors that determine cell performance. Reactant gases (hydrogen, oxygen) from the gas flow paths of the separators 98 are supplied to the catalyst layer (CL) 95 through the gas diffusion layers (GDL) 97 and porous layers (MPL) 96. Water produced is discharged along the reverse path.
An example of a separator is one in which flow paths are formed by machining a graphitized carbon plate treated so as to be impermeable to gas. Although a separator of this kind exhibits excellent performance in terms of electrical conductivity, corrosion resistance and reliability, it is difficult to make the separator more compact and the cost thereof is two orders of magnitude higher than that required for mass production of fuel cell vehicles (FCV) and the like. Development of a low-cost alternative is essential.
Heat/pressure-molded products of carbon material/resin composites and metal-molded products subjected to treatment for surface corrosion resistance have been proposed thus far. However, it is difficult to achieve both a thin film and mechanical strength with the former. In the case of the latter, the formation of electrically conductive protrusions in the oxide film on a stainless steel surface, plating with noble metal and cladding with corrosion-resistant metal have been attempted. Nevertheless, although compactness is satisfactory, major issues remain in terms of corrosion resistance and cost.
In any case, groove-shaped gas flow paths are formed in the separator surfaces, as illustrated in FIG. 35. Even if the separator itself is a simple flat metal plate, a body formed to have groove-shaped gas flow paths is joined to the flat metal plate (for example, see Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-Open No. 2011-150801
Thus, with a structure in which the cathode gas or anode gas is supplied through groove-shaped gas flow paths, these gases are localized along the flow paths. Accordingly, a gas diffusion layer or the like for uniformly diffusing these gases toward the catalyst layers and electrolyte membrane is essential. A gas diffusion layer comprising a carbon fiber material or the like is a cause of higher cost.